The present invention relates generally to thermocouples, and more particularly, to thermocouple junction boxes for high-temperature environments.
Accurate temperature measurement is often a challenge for instrument engineers, due, in part, to advanced technologies requiring more and more accurate temperature measurements. Traditionally, when high accuracy in temperature readings are important, resistance temperature detectors (RTDs) are often chosen over thermocouples for temperature sensing. However, RTDs offer a limited temperature range and are much more sensitive to vibration and mechanical shock. Thus, there are circumstances where the thermocouple is the only possible choice for temperature measurements. However, as accuracy requirements become more important, it is increasingly important that care is taken in the design and maintenance of thermocouple circuits to assure the most accurate sensing possible.
Thermocouples by nature have inherent problems. First of all, the inaccuracies of thermocouples, due to variables in the alloys of metals used in manufacturing, are considerable with most calibrations. Secondly, many thermocouples have extension wires between the thermocouple sensor and the instrumentation used for indication or control. This adds additional error to the circuit. Also, once the thermocouple signal reaches the measuring instrument, the signal requires a stable, known reference junction. Further, thermocouples are subject to secondary junctions and ground loops, as well as electrical noise.
One of the major sources of error in thermocouple circuits are secondary junctions. Secondary junctions may be formed at connection points in the thermocouple extension wire, bends or kinks in the wire, or abrasions in fiberglass or ceramic fiber wire insulators that expose the thermocouple conductors.
Several methods have been employed in the prior art to limit the amount of error introduced into thermocouple readings by secondary junctions. These solutions have focused on minimizing the number of connection points in the thermocouple extension wire, using one run or length of wire to avoid terminal blocks and connectors, ensuring that the thermocouple extension wire makes no sharp bends and is free of kinks that may cause a short which would result in a secondary junction, and avoiding abrasion of the fiberglass or ceramic insulators, which exposes the conductors. When connections are necessary, a known prior art method of minimizing error involves matching the alloys of the connector or terminal block to the alloy of the thermocouple. Another prior art error reduction method is to ensure that any connection in the circuit is made in an area of relatively constant temperature to avoid temperature gradients across connection points.
However, the prior art solutions have failed to address the problem of EMF generated by studs when using stud-and-cup components. FIGS. 3, 4 and 5 illustrate a prior art thermocouple 400, including junction box 410 having a typical stud-and-cup configuration. FIGS. 6 and 7 are respectively a wiring diagram and a current flow diagram for the prior art thermocouple. This thermocouple 400 comprises two conductors 200, 201 connected at a primary junction 415, one conductor 200 formed of KP metal, and the other 201 formed of KN metal, each being welded to a pigtail 202. Each pigtail 202 is connected to a type K cup 220, through which a threaded stainless steel stud 203 is disposed and to which the stud 203 is connected. Each stud 203 is adapted to receive a corresponding washer or lug 204 and nut 206 threadably disposed onto the stud 203, for attachment of the pigtail 202 and cup 220 to the thermocouple cable 205 (which is connected to the washer or lug 204). In this prior art thermocouple, the stainless steel stud is capable of generating EMF, thus introducing error into the circuit.
The error created by the above-described prior art configuration was the subject of a recent experiment by the inventor. When the stainless steel studs of several prior art thermocouple junction boxes were exposed to 300° F. blowing air while the tip of the thermocouple was kept at room temperature, an amount of error significantly larger than the error caused by typical secondary junctions, i.e., outside of the typical tolerances, was introduced into the measurement system. It is the belief of the inventor that the stainless steel stud is a major contributor to the overall measurement error in thermocouples that employ such a stud-and-cup design, and improvement of junction box design is needed to improve the accuracy of the thermocouple.